projects: project-wide symbol search over a SQLite FTS5 index (phase 2)

idatui/index.py — one on-disk index (<sidecar>/idx/project.db) over every binary
in a project, so search works for binaries whose worker isn't running.

Indexing choice, measured rather than guessed:
* SQLite FTS5 with the TRIGRAM tokenizer — stdlib, no dependency (nothing else
  was installed and nothing is needed), and unlike a prefix index it matches
  arbitrary substrings, which is what symbol names and string bodies need.
* 300k-entry corpus: 1.9 ms per query vs 11.8 ms for a Python scan and 28.9 ms
  for plain LIKE; 0.2 ms per incremental insert.
* Size was the stated worry and turned out not to bite: bash contributes 5.9k
  entries / 0.15MB of text, libcrypto.so.3 30.7k / 0.52MB. At ~5.7x the text a
  20-binary project is ~12-23MB — against .i64 files already in the sidecar
  (libcrypto's alone is 72MB), roughly 1% of what the project already costs. The
  reason to be on disk is residency, not size.
* Trigram can't answer queries under 3 chars and returns nothing rather than
  erroring, so search() falls back to LIKE — otherwise incremental typing would
  look broken until the third keystroke.

Wiring: after a binary's functions load, its symbols + strings are folded into
the index (skipped when the source's size/mtime is unchanged). Ctrl+N gains a
scope toggle on F2 — not ctrl+a, which the focused Input binds to "home" so it
never reaches the palette. Project scope narrows via the index then ranks with
the existing _fuzzy, keeping the same feel; hits are prefixed with their binary,
and choosing one elsewhere switches binary and jumps to it.

Also fixes another instance of the Textual-markup trap: the palette titles ate
"[project]" as a style tag (same class of bug as the status bar), so the pal
titles are markup=False now.

tests/test_index.py: 24 stdlib checks — substring/case-insensitive matching, kind
filter, the <3 char fallback, multi-binary search, per-binary incremental
reindex, staleness, forget, persistence. Suite 191/0.

Strings (") still needs the same scope toggle; the index already carries them.

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